Organic electroluminescent (hereinafter written as “EL”) elements utilizing electroluminescence of organic materials (also referred to as “organic light-emitting diodes”) have been practically used as a new generation of light-emitting systems enabling planar light emission. The organic EL elements have been applied to not only electronic displays but also lighting apparatuses, and their development has been expected.
The prototype of organic EL elements (composed of fluorescent materials) was published in 1987. In 1998, an organic EL element of another type composed of a phosphorescent material was published that utilizes all of singlet excitons and triplet excitons in electroluminescence to attain high luminescent efficiency.
Phosphorescent organic EL elements emitting green light and red light have been developed as a result of development of materials and techniques attaining the luminescent efficiency and the emission lifetime of practical levels, and have been used in electronic displays and lightings.
Phosphorescent organic EL elements emitting blue light should be composed of a luminous material having higher triplet energy than that of the luminous materials for red and green light emissions, and thus the luminous material for blue light emission should have a wide gap between the energy level of the ground state and that of the excited state. Such requirements reduce molecular stability, resulting in luminescent elements having shorter service lives (for example, see NPL 1).
A wider gap between the energy level of the ground state and that of the excited state is required to attain light emission of a color closer to pure blue or blue violet. Alight emission having such a shorter wavelength is completely traded off against a longer service life of the luminescent element, and the compatibility therebetween has not been established yet.
Typical luminous materials for organic EL elements include the phosphorescent materials and fluorescent materials. Anew trend has been found in the development of the fluorescent materials.
For example, PTL 1 focuses attention on a phenomenon of generation of a singlet exciton through fusion of two collided triplet excitons (hereinafter referred to as “triplet-triplet fusion (TTF) phenomenon”), and discloses a technique of efficiently generating the TTF phenomenon to enhance the efficiency of the fluorescence elements. The blue fluorescent material attained by this technique has double or triple the power efficiency of the traditional blue fluorescent materials. Unfortunately, the fluorescent materials intrinsically have a conversion rate of 50% from the lowest excited triplet energy level to the lowest excited singlet energy level. In other words, the luminescent efficiency of the fluorescent materials is limited in principle and cannot be enhanced to be equal to the luminescent efficiency of the phosphorescent materials.
PTL 2 discloses a technique using a delayed fluorescent compound composed of a copper complex to enhance luminescent efficiency. Unfortunately, the compound disclosed in PTL 2 has light emissions ranging from green to red. The luminescent efficiency of the compound is higher than that of the traditional fluorescent materials, but is significantly lower than that of the phosphorescent materials having high luminescent efficiency.
Delayed fluorescent compounds having a different light-emitting scheme called thermally-activated delayed fluorescent compounds (hereinafter also referred to as TADF compounds) have been found, and their applications to organic EL elements have been examined (for example, see NPLs 2 to 7 and PTL 3).
The TADF compounds have a unique light-emitting mechanism as illustrated in an energy diagram in FIG. 1. The difference between the lowest excited singlet energy level and the lowest excited triplet energy level in the TADF compounds is smaller than that in typical fluorescent materials (ΔEst(TADF) is smaller than ΔEst(F) in FIG. 1). Electric field excitation generates triplet excitons at a 75% probability, and these triplet excitons usually do not contribute to light emission. If the difference between the lowest excited singlet energy level and the lowest excited triplet energy level is small enough, the triplet excitons transit to a singlet excited state under heat of the organic EL element being driven, and are radiatively deactivated (also referred to as “radiative transit” or “radiative deactivation”) from the singlet excited state to the ground state to generate fluorescence.
The lowest excited triplet energy T1 (TADF) is always smaller (or more stable) than the lowest excited singlet energy S1(TADF) even in this transition. Such a relation between T1 (TADF) and S1(TADF) prevents part or most of the triplet excitons generated at a 75% probability to transit to the singlet excited state through intersystem crossing, so that part or most of the triplet excitons generated at a 75% probability are thermally deactivated. Moreover, the lifetime of excitation at T1 longer than that at S1(TADF) and agglomeration of the TADF compounds in films formed through deposition readily cause triplet-triplet (T-T) annihilation to prevent an enhancement in luminescent efficiency.
NPL 2 attempted to enhance luminescent efficiency through modification of substituents to effectively utilize the triplet excitons (T1(TADF)) of TADF compounds subjected to thermal deactivation or T-T annihilation. Unfortunately, a reduction in the difference between the lowest excited singlet energy level and the lowest excited triplet energy level has technical difficulties in molecular design and synthesis. Actually, introduction of a substituent into a TADF compound for the dispersion of the TADF compound in films leads to a significant reduction in luminescent efficiency, change in wavelength of light emitted, and a reduction in driving life of elements, which phenomena are not found in the common fluorescent materials and phosphorescent materials.